“Despite repeated requests and talks with the chief constable’s office, the HET and the Policing Board there is no sign of the report being given to those most affected, the bereaved families,” said the firm.

“We can confirm that, as of today, the families have instructed us to commence legal proceedings in the High Court in Belfast to get access to the report.

“They do so reluctantly but have no choice.”

The families have also lodged a complaint with the Police Ombudsman’s Office.

Later on Thursday, some of the families of those who were killed will lodge a complaint in person with the Policing Board.

In February 2011, the Police Ombudsman published a report into the attack claiming the original police investigation was “biased”.

It said the RUC was so focused on the idea that the IRA was responsible that it did not properly probe loyalist involvement.